1. Field of the Invention
The present invention relates to a method for incorporating disk media into a magnetic disk drive used as external storage for a computer or the like. Particularly, the present invention relates to a method for incorporating disk media into a magnetic disk drive after servo information has been written onto the disk medium for positioning a head of the magnetic disk drive at an aimed track in the disk medium.
2. Description of the Related Art
In the prior art, the operation for writing servo information onto a disk medium for positioning a head at an aimed track of a disk medium (hereinafter referred to as "servo track writing (STW) operation") is performed after the disk medium has been incorporated into the magnetic disk drive. In this regard, there is a problem in that a spindle and a head actuator may radially oscillate in the STW operation (hereinafter referred to as "runout (RO)"). This runout (RO) includes one occurring in synchronism with the rotation of the disk media (repeatable runout (RRO)) and another occurring independent from the rotation of the disk media (non-repeatable runout (NRRO)). Among them, the NRRO is particularly serious when the track density becomes higher.
FIG. 11 is an illustration for explaining the runout when the STW operation is being performed in the prior art. If the runout occurs when the servo information is being written onto the disk medium 10, the servo track information is written in a meandering manner in the radial direction. If the runout consists solely of the RRO, all the tracks are meandering while keeping the parallelism therebetween as shown in the upper left area of FIG. 11, which is no problem provided that the head could follow these tracks when data are written and read. However, if NRRO is added, the servo information is written while randomly deviated in the radial direction as shown in the lower left area of FIG. 11, whereby data are recorded based on the servo information written in such a deviated manner. When NRRO is significant in such a case, there might be interference between data in the adjacent tracks.
However, since there are limitations in cost, size and structure of the spindle and the head actuator incorporated into the device, it is impossible to design the device to completely minimize NRRO.
A ball bearing is often used as a bearing for a spindle motor to be incorporated into the device. One of primary factors for causing the NRRO in a ball bearing is a manufacturing accuracy of the parts thereof. Although it is, of course, possible to have high-accuracy parts, the manufacturing cost thereof is very expensive. A fluid bearing wherein a space between a stationary part and a rotary part is filled with fluid is recommended as a bearing capable of minimizing the NRRO. However, it is necessary for this purpose to increase the stiffness thereof by reducing the space between the stationary part and the rotary part or increasing the viscosity of the fluid, both resulting in an increase in working torque or loss torque.
Since a head actuator to be incorporated in a magnetic disk drive must move at a high speed on a disk medium to write and read data, mass and moment of inertia of a movable part of the head actuator should necessarily be as small as possible to be capable of high-speed running. However, if mass and moment of inertia become too small, the movable part is liable to be adversely affected by disturbance such as an air stream occurring when the disk medium rotates, resulting in oscillation.
Accordingly, the present invention relates to one which employs a system wherein the disk media are incorporated into the spindle of the magnetic disk drive after the servo information has been written onto an individual magnetic disk or a stack of magnetic disks. Such a system wherein the servo information is written onto a separate magnetic disk is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 3-73406, as the title of "Servo Information Writing Method for in a Disk drive", which has its purpose to reduce a loss due to the inferiority of a magnetic disk medium or that of a driving system for rotating a magnetic disk drive to enhance the versatility of a device for writing a servo track information, so that a low cost magnetic disk drive is realized.
Thereby, it is possible to use, during the STW operation, a spindle having a minimum NRRO, for example, one incorporating a high-accuracy static pressure air bearing. Also, a head actuator for the STW operation is not necessarily driven at a high speed and thus may have a larger mass so as not to be influenced by a disturbance. Accordingly, it is possible to minimize the NRRO during the STW operation, whereby the servo track information can be written onto the disk medium at a higher track density.
FIGS. 12(a) and 12(b) illustrate an example of the conventionally known system wherein a disk medium is incorporated into a magnetic disk drive after the servo information has been written onto the disk medium. In FIG. 12(a), the servo information is being written, by a STW head actuator 13, onto a single disk medium 10 set on a STW spindle 11 by a clamper 12. On the other hand, in FIG. 12(b), a plurality of disk media 10 are being mounted on a spindle 15 secured to a housing 14 of a magnetic disk drive by a clamper 16.
The present invention may also be applicable to other systems for writing the servo information on the magnetic disk prior to the incorporation thereof into the disk drive than that described above. A system for forming a servo pattern by a magnetic layer includes one wherein the servo pattern is recorded by forming an irregularity on the magnetic layer (see Japanese Unexamined Patent Publication (Kokai) No. 6-68444 as a title of "Method and Apparatus for Writing Positioning Signals") and one wherein the servo pattern is recorded by pattern-etching the magnetic layer (see prior Patent Application No. 7-325320 filed by the same assignee of this application).
In a system wherein a single, separate disk medium is incorporated into the magnetic disk drive after the servo information is recorded onto the separate disk medium, it is important to match the rotational center of the disk medium during the STW operation with that of the disk medium during the incorporation thereof into the disk drive, or to match the center of servo pattern formation on the magnetic layer with that of the disk medium during the incorporation thereof into the disk drive.
FIG. 13 is the illustration for explaining a problem which may occur during the incorporation of the disk medium 10 into a spindle hub 15 of the disk drive.
For example, the track pitch would be 1.5 .mu.m if one wishes to realize a high track density of 17,000 TPI (tracks per inch). Assuming that it would be possible to suppress a component of runout (RO) due to the rotation to a level of -40 dB or less by a servo control technique for a head, the RO due to a spindle motor must be at most .+-.3.75 .mu.m for the purpose of suppressing the rotational component of RO due to the rotation of the spindle motor at a level of .+-.2.5% of the track pitch.
However, since a tolerance of an inner diameter of the disk medium would be 50 .mu.m, if the disk medium 10 was incorporated into the spindle hub 15 without any special attention, the rotation center of the disk medium 10, when the STW is performed, would be deviated from that when the disk medium 10 is incorporated into the disk drive by 50 .mu.m at maximum. Therefore, it should be considered that the RO is 50 .mu.m at maximum. In addition, if the tolerance of the spindle hub 15 was included, the deviation would be even greater.
If the tolerance of the inner diameter of the disk medium 10 was reduced, the cost for making the disk medium 10 would significantly be increased. Therefore, it is impossible to further reduce the tolerance. Accordingly, in order to match the center of the disk medium when the servo track information is written onto the disk medium (STW) with the center thereof when the disk medium is incorporated into the disk apparatus as closely as possible, a method for incorporating the disk medium into the disk apparatus must be improved.
A method for forming servo patterns with a magnetic film has the same situation as mentioned above and, therefore, a similar improvement is required also for this case.